The present invention is directed generally to data processing systems, and more particularly to a multiple processing system and a reliable system area network that provides connectivity for interprocessor and input/output communication. Further, the system is structured to exhibit fault tolerant capability.
Present day fault tolerant computing evolved from specialized military and communications systems to general purpose high availability commercial systems. The evolution of fault tolerant computers has been well documented (see D. P. Siewiorek, R. S. Swarz, "The Theory and Practice of Reliable System Design," Digital Press, 1982, and A. Avizienis, H. Kopetz, J. C. Laprie, eds., "The Evolution of Fault Tolerant Computing," Vienna: Springer-Verlag, 1987). The earliest high availability systems were developed in the 1950's by IBM, Univac, and Remington Rand for military applications. In the 1960's, NASA, IBM, SRI, the C. S. Draper Laboratory and the Jet Propulsion laboratory began to apply fault tolerance to the development of guidance computers for aerospace applications. The 1960's also saw the development of the first AT&T electronic switching systems.
The first commercial fault tolerant machines were introduced by Tandem Computers in the 1970's for use in on-line transaction processing applications (J. Bartlett, "A NonStop Kernal," in proc. Eighth Symposium on Operating System Principles, pp. 22-29, December 1981). Several other commercial fault tolerant systems were introduced in the 1980's (O. Serlin, "Fault-Tolerant Systems in Commercial Applications," Computer, pp. 19-30, August 1984). Current commercial fault tolerant systems include distributed memory multi-processors, shared-memory transaction based systems, "pair-and-spare" hardware fault tolerant systems (see R. Freiburghouse, "Making Processing Fail-safe," Mini-micro Systems, pp. 255-264, May 1982; U.S. Pat. No. 4,907,228 is also an example of this pair-and-spare technique, and the shared-memory transaction based system.), and triple-modular-redundant systems such as the "Integrity" computing system manufactured by Tandem Computers Incorporated of Cupertino, Calif., assignee of this application and the invention disclosed herein.
Most applications of commercial fault tolerant computers fall into the category of on-line transaction processing. Financial institutions require high availability for electronic funds transfer, control of automatic teller machines, and stock market trading systems. Manufacturers use fault tolerant machines for automated factory control, inventory management, and on-line document access systems. Other applications of fault tolerant machines include reservation systems, government data bases, wagering systems, and telecommunications systems.
Vendors of fault tolerant machines attempt to achieve both increased system availability, continuous processing, and correctness of data even in the presence of faults. Depending upon the particular system architecture, application software ("processes") running on the system either continue to run despite failures, or the processes are automatically restarted from a recent checkpoint when a fault is encountered. Some fault tolerant systems are provided with sufficient component redundancy to be able reconfigure around failed components, but processes running in the failed modules are lost. Vendors of commercial fault tolerant systems have extended fault tolerance beyond the processors and disks. To make large improvements in reliability, all sources of failure must be addressed, including power supplies, fans and intermodule connections.
The "NonStop," and "Integrity" architectures manufactured by Tandem Computers Incorporated, (both respectively illustrated broadly in U.S. Pat. No. 4,228,496 and U.S. Pat. Nos. 5,146,589 and 4,965,717, all assigned to the assignee of this application; NonStop and Integrity are registered trademarks of Tandem Computers Incorporated) represent two current approaches to commercial fault tolerant computing. The NonStop system, as generally shown in the above-identified U.S. Pat. No. 4,278,496, employs an architecture that uses multiple processor systems designed to continue operation despite the failure of any single hardware component. In normal operation, each processor system uses its major components independently and concurrently, rather than as "hot backups". The NonStop system architecture may consist of up to 16 processor systems interconnected by a bus for interprocessor communication. Each processor system has its own memory which contains a copy of a message-based operating system. Each processor system controls one or more input/output (I/O) busses. Dual-porting of I/O controllers and devices provides multiple paths to each device. External storage (to the processor system), such as disk storage, may be mirrored to maintain redundant permanent data storage.
This architecture provides each system module with self-checking hardware to provide "fail-fast" operation: operation will be halted if a fault is encountered to prevent contamination of other modules. Faults are detected, for example, by parity checking, duplication and comparison, and error detection codes. Fault detection is primarily the responsibility of the hardware, while fault recovery is the responsibility of the software.
Also, in the Nonstop multi-processor architecture, application software ("process") may run on the system under the operating system as "process-pairs," including a primary process and a backup process. The primary process runs on one of the multiple processors while the backup process runs on a different processor. The backup process is usually dormant, but periodically updates its state in response to checkpoint messages from the primary process. The content of a checkpoint message can take the form of complete state update, or one that communicates only the changes from the previous checkpoint message. Originally, checkpoints were manually inserted in application programs, but currently most application code runs under transaction processing software which provides recovery through a combination of checkpoints and transaction two-phase commit protocols.
Interprocessor message traffic in the Tandem Nonstop architecture includes each processor periodically broadcasting an "I'm Alive" message for receipt by all the processors of the system, including itself, informing the other processors that the broadcasting processor is still functioning. When a processor fails, that failure will be announced and identified by the absence of the failed processor's periodic "I'm Alive" message. In response, the operating system will direct the appropriate backup processes to begin primary execution from the last checkpoint. New backup processes may be started in another processor, or the process may be run with no backup until the hardware has been repaired. U.S. Pat. No. 4,817,091 is an example of this technique.
Each I/O controller is managed by one of the two processors to which it is attached. Management of the controller is periodically switched between the processors. If the managing processor fails, ownership of the controller is automatically switched to the other processor. If the controller fails, access to the data is maintained through another controller.
In addition to providing hardware fault tolerance, the processor pairs of the above-described architecture provide some measure of software fault tolerance. When a processor fails due to a software error, the backup processor frequently is able to successfully continue processing without encountering the same error. The software environment in the backup processor typically has different queue lengths, table sizes, and process mixes. Since most of the software bugs escaping the software quality assurance tests involve infrequent data dependent boundary conditions, the backup processes often succeed.
In contrast to the above-described architecture, the Integrity system illustrates another approach to fault tolerant computing. Integrity, which was introduced in 1990, was designed to run a standard version of the Unix ("Unix" is a registered trademark of Unix Systems Laboratories, Inc. of Delaware) operating system. In systems where compatibility is a major goal, hardware fault recovery is the logical choice since few modifications to the software are required. The processors and local memories are configured using triple-modular-redundancy (TMR). All processors run the same code stream, but clocking of each module is independent to provide tolerance of faults in the clocking circuits. Execution of the three streams is asynchronous, and may drift several clock periods apart. The streams are re-synchronized periodically and during access of global memory. Voters on the TMR Controller boards detect and mask failures in a processor module. Memory is partitioned between the local memory on the triplicated processor boards and the global memory on the duplicated TMRC boards. The duplicated portions of the system use self-checking techniques to detect failures. Each global memory is dual ported and is interfaced to the processors as well to the I/O Processors (IOPs). Standard VME peripheral controllers are interfaced to a pair of busses through a Bus Interface Module (BIM). If an IOP fails, software can use the BIMs to switch control of all controllers to the remaining IOP. Mirrored disk storage units may be attached to two different VME controllers.
In the Integrity system all hardware failures are masked by the redundant hardware. After repair, components are reintegrated on-line.
The preceding examples illustrate present approaches to incorporating fault tolerance into data processing systems. Approaches involving software recovery require less redundant hardware, and offer the potential for some software fault tolerance. Hardware approaches use extra hardware redundancy to allow full compatibility with standard operating systems and to transparently run applications which have been developed on other systems.
Thus, the systems described above provide fault tolerant data processing either by hardware (e.g, fail-functional, employing redundancy) or by software techniques (fail-fast, e.g., employing software recovery with high data integrity hardware). However, none of the systems described are believed capable of providing fault tolerant data processing, using both hardware (fail-functional) and software (fail-fast) approaches, by a single data processing system.
Computing systems, such as those described above, are often used for electronic commerce: electronic data interchange (EDI) and global messaging. Today's demands upon such electronic commerce, however, is demanding more and more throughput capacity as the number of users increases and messages become more complex. For example, text-only e-mail, the most widely used facility of the Internet, is growing significantly every year. The Internet is increasingly being used to deliver image, voice, and video files. Voice store-and-forward messaging is becoming ubiquitous, and desktop video conferencing and video-messaging are gaining acceptance in certain organizations. Each type of messaging demand successively more throughput.
In such environments, parallel architectures are being used, interconnected by various communication networks such as local area networks (LANS), and the like.
A key requirement for a server architecture is the ability to move massive quantities of data. The server should have high bandwidth that is scalable, so that added throughput capacity can be added as data volume increases and transactions become more complex.
Bus architectures limit the amount of bandwidth that is available to each system component. As the number of components on the bus increases less bandwidth is available to each.
In addition, instantaneous response is a benefit for all applications and a necessity for interactive applications. it requires very low latency, which is a measure of how long it takes to move data from the source to the destination. Closely associated with response time, latency affects service levels and employee productivity.